1. Field of the Invention
The present invention relates to a method for preparing an unsaturated carboxylic acid using an ammonium salt of hydroxycarboxylic acid.
2. Description of the Related Art
Acrylic acid is an unsaturated carboxylic acid, and polyacrylic acid is widely used for preparing water-absorbing resins. Currently, such acrylic acid is prepared via a two-step oxidation process of propylene in the presence of an oxide catalyst. Propylene, as the raw material of acrylic acid, can be obtained from petroleum, but the price of propylene is rising due to increased demand for petroleum.
In order to prepare acrylic acid independent of petroleum, dehydration of biomass-derived lactic acid or 3-hydroxypropionic acid to acrylic acid has been reported. In addition, conversion of lactate or 3-hydroxypropionate produced by fermentation of biomass into acrylic acid has been studied.
U.S. Pat. No. 2,469,701 discloses a method for preparing acrylic acid by supplying 3-hydroxypropionic acid (3HP) in a reactor containing a dehydration catalyst under heating and reduced-pressure conditions. However, there is no mention of conversion of 3-hydroxypropionate generated during fermentation into acrylic acid.
U.S. Patent Publication No. 2005-0222458 discloses a method for preparing acrylic acid by heating 3-hydroxypropionic acid or its derivatives obtained from microbial or plant cells. Specifically, acrylic acid is prepared by vapor-phase dehydration of 3-hydroxypropionic acid in the presence of a heterogeneous catalyst, such as silica, alumina, or titania. When the heterogeneous catalyst is used, acrylic acid can be obtained in a high yield, but there is the problem of catalyst contamination due to organic impurities, which reduces catalyst performance.
U.S. Patent Publication No. 2005-0221457 discloses a method for preparing a salt (or ester) of acrylic acid by heating the salt (or ester) of 3-hydroxypropionic acid. However, this method concerns only the conversion of sodium and calcium salts of 3-hydroxypropionic acid at elevated pressure, and the yield of acrylic acid by dehydration is poor (41.8%, 48.7%).
U.S. Pat. No. 8,198,481 discloses a method for preparing acrylic acid by a two-step reaction of 3-hydroxypropionic acid and phosphoric acid. However, the two-step reaction adds cost to the overall process by requiring additional unit operations and equipment, as well as by generating additional wastes.
Therefore, there remains a desire for a method for preparing an unsaturated carboxylic acid in a simple and economic manner and in a high yield.